Knee arthrodesis consists in combining and joining by bone reconstruction the proximal ends of the tibia after resection of the epiphyses.
This operation is advised in the case of severe, in particular infectious, complications following the fitting of articulated prostheses or following major trauma or bone ablations for the treatment of malignant tumours preventing the fitting of prostheses.
The principle of an arthrodesis with the aid of an implantable device consists in implanting a femoro-tibial immobilising material in the metaphyses and medullary parts of the femur and the tibia allowing the immobilised bones to be supported during reconstruction of the peripheral bone and when an intercalary bone graft is carried out.
This implantable immobilising material traditionally consists of an intramedullary nail (Juvara nail) of small diameter, typically 12 mm, allowing maximum bone reconstruction around this nail and of considerable length to be anchored into the medullary canal of the femur and the tibia. However, these nails pose problems with their fitting and with their shapes which are not adapted to natural shapes—in particular to the valgus or flexion angles and the curvature of the femur.
To remedy these problems, FR-2 800 987 has proposed a made-to-measure, two-element implantable device which allows a surgical operation of short duration and causes less post-operative trauma than an intramedullary nail. This device comprises a femoral implant with an intramedullary femoral rod and a tibial implant with an intramedullary tibial rod, in addition to a rigid part connecting the proximal ends of these two implants relative to one another, vertically to the joint. Each intramedullary rod comprises a first distal section of small diameter, a second intermediate section which is slightly in the shape of a truncated cone and a third proximal section of constant section of larger diameter. The rigid connecting part is inserted between the two implants with which it interlocks. It has a cylindrical body with bores manufactured to create the valgus and flexion angles and two anti-rotating radial blades.
This implantable device has the disadvantage of having to be made to size prior to each patient's operation. During the implantation, the quality of the bone of the patient may be worse than expected in view of the examinations and x-rays previously carried out. In fact, arthrodesis is generally a solution considered after multiple operations, the failure of successive prosthetic solutions or following serious trauma to the knee. The implantation site has often been the subject of an infection or multiple infections, having deteriorated the bone tissue, in particular in the region of the proximal ends of the bones. The implantable device prefabricated to size, disclosed by FR-2 800 987, cannot then be fixed. The slightly conical intermediate section of the rods which rests on the medullary canal offers insufficient support and is not axially wedged with sufficient precision in the bone. Despite the fact that the proximal ends of the implants are as unobstructive as possible in the radial direction to facilitate the femoro-tibial peripheral bone reconstruction, this bone reconstruction often proves to be problematical.
In practice, the inventor has established that failures encountered with the aforementioned known implantable device actually stem from insufficient wedging of the implants relative to the bones, from a poor adaptation of the size of the implants in spite of being manufactured to size and from insufficient support of the two bones relative to one another, in particular in torsion.
U.S. Pat. No. 5,108,398 also discloses an implantable device for arthrodesis, of controllable length, comprising a femoral rod, a tibial rod and a screwed-on cylinder for controlling the length of the femoral rod. Here again, the inventor has established that these implants are not sufficiently well wedged relative to the bones and do not create sufficient support, in particular in torsion, in spite of the presence of distal transverse screws.
It is noteworthy moreover that an implantable device for arthrodesis is an orthesis which, in contrast to a prosthetic implant, has the primary purpose of supporting post-operative ankylosis for a sufficient duration to allow the bone reconstruction of the graft between the femur and the tibia (such as external fixings or previously used Juvara nails). In view of this ankylosis it is inconceivable for the implant to withstand all the stresses of walking. Whilst the implantable device of FR-2 800 987 allows the patient to be upright before the end of the reconstruction, it is not designed to withstand all the mechanical stresses connected with walking in the long term. The necessity of this peripheral osteofusion and the specific problems connected to the desired ankylosis mean that the solutions considered for partially or totally articulated prostheses designed to replace all or part of the joint, by ensuring mobility and by withstanding over the long term the mechanical stresses of walking, are badly adapted and cannot be transferred to ortheses for arthodesis. In particular, in an articulated prothesis, the problem of radial obstruction of the implants at the proximal ends does not occur.
DE-19 722 389 has proposed a device for arthrodesis formed in the manner of a secured prothesis of which the condyles are replaced by fixed plates on intramedullary rods which have symmetry of revolution, these plates axially bearing against the resected proximal ends of the bones, the two plates being attached to one another by a dovetail connection. This device prevents any peripheral bone reconstruction and is singlehandedly supposed to withstand all the long-term femoro-tibial mechanical stresses, which never occur in the case of ankylosis. In fact the two immobilised bones generate between them extremely large mechanical stresses, in particular in flexion and in torsion, and in any case much greater than when a possibility of movement is retained. The device disclosed in this document is destined to break and in any case does not provide anchoring of the implants in the bones which is compatible with maintaining ankylosis, despite the specific implant/bone contact structures recommended by this document. Radial movement is certain to be observed between the implants and the bones in the short or medium term after implantation. The impossibility of using the device of DE-19 722 389 in reality is therefore a practical illustration of how the solutions considered for the protheses cannot be applied in the case of arthrodeses.